C1-C8 carboxylic acid salt solution for the absorption of CO2

ABSTRACT

The invention provides a method for the capture of at least one acid gas in a composition, the release of said gas from said composition and the subsequent regeneration of said composition for re-use. The method comprises the step of capturing an acid gas by contacting said acid gas with a capture composition comprising at least one salt of a carboxylic acid dissolved in a solvent system consisting substantially of water.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a 35 U.S.C. § 371 U.S. national phase entry ofInternational Application No. PCT/GB2018/052209 having an internationalfiling date of Aug. 2, 2018, which claims the benefit of Great BritainApplication No. 1712465.2 filed Aug. 2, 2017, each of which isincorporated herein by reference in its entirety.

FIELD OF THE INVENTION

The present invention is concerned with a method for the capture andsubsequent release of acid gases such as carbon dioxide. Captureinvolves contacting the gas with an aqueous solution of a carboxylatesalt.

BACKGROUND TO THE INVENTION

As a result of the increasing use of fossil fuels, the concentration ofcarbon dioxide in the atmosphere has risen from 280 ppm inpre-industrial times, to almost 400 ppm in 2013, leading to rises inaverage global temperatures. This is expected to increase further in theshort- to mid-term until energy supplies which do not result insignificant CO₂ emissions become established. According to theInternational Energy Agency World Energy Outlook (2002), the predictedincrease in combustion generated CO₂ emissions is around 1.8% per yearand by 2030, if it continues at that rate, it will be 70% above 2000levels.

Current methods for capture of CO₂ and other acid gases are expensiveand far from ideal for large scale application, so the present inventionattempts to address this problem by providing a solution which isrelatively simple, and uses inexpensive processes and consumables,designed to minimise the overall environmental impact of large scaledeployment.

WO2015/092427 discloses a methodology for the capture and release of CO₂and other acid gases using carboxylate salts are dissolved in a mixtureof a protic solvent (e.g. water) and a non-aqueous organic solvent. Theinventors identified that the presence of a non-aqueous solventenvironment increases the pKa of carbonic acid (or more specifically,aqueous CO₂) to a much lesser degree than it does the pKa of thecarboxylic acid, meaning that the carboxylate ions become effectivecapture agents. This allows exploitation of non-conventional captureagents under non-conventional conditions to eliminate or ameliorate manyof the peripheral disadvantages associated with the currentstate-of-the-art capture technologies but most critically reduces theenergy input requirement for the full capture and release cycle tolevels not considered possible up to now.

The use of the non-aqueous solvent presents problems from a practicalpoint of view that could limit the utility of this technology. Thesolvents tend to have moderate volatility, meaning that replenishment isneeded, the solvents tend to be combustible, leading to additionalsafety measures and the solvents can undergo a small amount ofdegradation as they are exposed to impurities, e.g. oxygen, in the gascomprising the acid gas. Nevertheless, it was thought, however, thatcarboxylate salts would be ineffective as capture agents without thenon-aqueous solvent because the pKa of the conjugate acid is too low forthem to react with aqueous CO₂ and carbonic acid to displace the carbondioxide hydration equilibrium.

SUMMARY OF THE INVENTION

In a first aspect of the present invention is provided a method for theeffective capture of an acid gas. The method comprises:

-   -   a. capturing an acid gas by contacting said acid gas with a        capture composition comprising at least one salt of a carboxylic        acid dissolved in a solvent system consisting substantially of        water.

The method may also comprise:

-   -   b. releasing said at least one acid gas by heating the loaded        capture composition, and/or by subjecting the loaded capture        composition to a stream of stripping gas, for example air,        and/or by reducing the pressure above the loaded capture        composition

The method may also comprise:

-   -   c. regenerating the capture composition by cooling and/or        increasing the pressure above the capture composition

The present invention derives from an appreciation on the part of theinventors that the pKa value of a given compound, which is a function ofthe stability of its conjugate base, can be dependent on the totalcharacteristics of the solvent environment in which the pKa is measured.

TABLE 1 Variation of the pKa of acetic acid in different solvents EntrySolvent pKa 1 Water 4.76 2 Methanol 9.7 3 Ethanol 10.3 4 DimethylSulfoxide 12.3 5 Dimethylformamide 13.3 6 tert-Butanol 14.2 71,2-Dichloroethane 15.5 8 Acetonitrile 23.51

It is a well-established principle that the pKa of a given compound canvary significantly when it is measured in different solvents (Table 1).The pKa values given in data tables are assumed to be at “infinitedilution” such that the intermolecular interactions between molecules ofthe compound being tested, and the contribution of the solute to theoverall solvent characteristics, can be ignored. In practice, thedilution of the compound being measured is dependent on the sensitivityof the measuring technique with milli- and micro-molar concentrationsbeing typical.

When considering solutions of compounds that are several orders ofmagnitude more concentrated than those used to measure the pKa valuesquoted in data tables, the contribution of the compound to the overallsolvent environment must be considered. For highly soluble compounds,the proportions of the final mixture may be such that they might moreappropriately be considered “solvent in X” rather than “X in solvent”(Table 2, solubility data fromhttps://en.wikipedia.org/wiki/Solubility_table, accessed 14 Jul. 2017).

TABLE 2 Mass percentages of saturated aqueous solutions of highlysoluble compounds Mass % Mass % Entry Compound Solute Water 1 AntimonyTrifluoride 81.62 18.38 2 Barium Perchlorate 77.06 22.94 3 CaesiumAcetate 91.00 9.00 4 Fructose 78.95 21.05 5 Indium(III) Bromide 85.1014.90 6 Lithium Chlorate 78.81 21.19 7 Rubidium Formate 84.71 15.29

The pKa value of a given compound can therefore be considered a dynamicproperty dependent on, amongst other factors, the concentration at whichthat compound is present in solution. Strictly speaking, the pKa valuesquoted in data tables are only accurate for the stated solvent and atthe concentration at which the value was measured. For more concentratedsolutions, different pKa values can be observed with correspondinglydifferent acid-base reactivity behaviour.

The present application exploits this phenomenon by employing aqueoussolutions of carboxylate salts as capture agents for carbon dioxide andother acid gases. Carboxylate salts would not normally be considered tobe effective capture agents given the relatively low pKa of theirconjugates acids (typically ˜5). However, for concentrated solutions,the pKa of carboxylate salts can be sufficiently high as to allow foreffective acid gas capture.

Critically for these formulations, when comparing them to other carbondioxide capture systems, the products of the capture reaction are abicarbonate salt and corresponding carboxylic acid (Scheme 1).Thermodynamically speaking this represents a more finely balancedsystem, compared to more conventional capture systems, which cansignificantly reduce the temperature, and therefore the energy input,required for effective release.

In principle, bicarbonates and other acid gas salts (and thecorresponding acid gases) will also change pKa values based upon thecomposition of the solution but, surprisingly, this does not occur tothe same degree as for carboxylate salts. It is the difference betweenthe relative changes in the pKa values of acid gases and carboxylatesalts that allows effective capture by these concentrated solutions.

Compared to the capture compositions described in WO2015/092427, acidgases can be released from the capture compositions of the presentinvention at a lower temperature, thus reducing the energy consumedduring the process. Typically, the compositions described inWO2015/09427 require heating to over 100° C. to release a significantportion of the CO₂ that has been captured. In contrast, the compositionsof the present invention can release a significant portion of the CO₂when heated to 80° C. or even lower.

It may be that the salt of a carboxylic acid is a metal, ammonium,phosphonium or sulfonium salt. It may be that the salt of a carboxylicacid is a metal salt. It may be that the salt of a carboxylic acid is asalt of an alkali metal such at lithium, sodium or potassium. It may bethat the salt of a carboxylic acid is a potassium salt. It may be thatthe salt of a carboxylic acid is an alkylammonium or arylammonium saltfor example a triethylammonium, tetramethylammonium, tetrabutylammonium,benzyltrimethylammonium, a choline salt, or a guanidinium salt. It maybe that the salt of a carboxylic acid is a combination of two or morecations.

Salts of aliphatic, aromatic, or heteroaromatic carboxylic acids (e.g.salts of aliphatic or aromatic carboxylic acids) may be used for thepurposes of the invention. Suitable aliphatic carboxylic acids may beselected from straight chained, branched or cyclic carboxylic acidswhich may be saturated or unsaturated, substituted or unsubstituted bysubstituent groups, heteroatoms, aromatic or heteroaromatic ringssystems. The carboxylic acid or each carboxylic acid may comprise asingle carboxylic acid group. Polycarboxylic acids, such as di-, tri- ortetra-carboxylic acids are also suitable, as are polymeric acids such aspolyacrylic and polymethacrylic acids and naturally derived biopolymericcarboxylic acids, for example alginic acid (from seaweed) and pectin(from plant cell walls). Salts of aromatic or heteroaromatic carboxylicacids, such as benzoic acid, are also suitable for the purposes of theinvention as are zwitterionic carboxylate salts, including betaine andassociated derivatives and homologues. Such salts may be in solution,slurry or dispersion.

It may be that the carboxylic acid is not an amino acid or eachcarboxylic acid is not an amino acid. It may be that the carboxylic aciddoes not comprise nitrogen or each carboxylic acid does not comprisenitrogen. It may be that the capture composition is substantially freeof amino acids. It may be that the capture composition is substantiallyfree of any organic compounds that comprise nitrogen. The term‘substantially free’ may be considered to mean that no more than 25% ofthe capture composition is amino acid or an organic compound thatcomprises nitrogen. It may mean that no more than 10% of the capturecomposition is amino acid or an organic compound that comprisesnitrogen. It may mean that no more than 5% of the capture composition isamino acid or an organic compound that comprises nitrogen. It may meanthat no more than 2% of the capture composition is amino acid or anorganic compound that comprises nitrogen. It may mean that no more than1% of the capture composition is amino acid or an organic compound thatcomprises nitrogen.

Typically, said salt is a salt of a C₁₋₂₀ aliphatic carboxylic acid,more typically a salt of C₁₋₈ aliphatic carboxylic acid. Said aliphaticcarboxylic acid may be straight chained or branched. Exemplary acidsinclude acetic acid, propionic acid, butyric acid, iso-butyric acid,pivalic acid and hexanoic acid. In certain specific embodiments, thesalt is a potassium salt of an aliphatic carboxylic acids, e.g.potassium salt of a C₁₋₈ aliphatic carboxylic acids, that may bestraight chained or branched.

It may be that said salt of a carboxylic acid is present in the finalcomposition at a level ≤80% w/w relative to the total composition

It may be that the at least one salt of a carboxylic acid is a singlesalt of a carboxylic acid. It may be that it is a mixture of two or moresalts of carboxylic acids. Where the salts are present as a mixture oftwo or more salts, it may be that the two or more salts will comprisethe same cationic counterion but be derived from different carboxylicacids and/or it may be that the two or more salts are derived from thesame carboxylic acid but with varying cationic counterions. It may be amixture of at least one salt of a C₁-C₄ aliphatic carboxylic acid, thatmay be straight chained or branched, and at least one salt of a C₅-C₆aliphatic carboxylic acids, that may be straight chained or branched. Itmay be that both salts are potassium salts of the indicated carboxylicacids.

Aliphatic carboxylic acids, that may be straight chained or branched,may be unsubstituted by substituent groups that include heteroatoms,e.g. atoms selected from O, N, S and P. Aliphatic carboxylic acids may,however, be substituted by a substituent group that comprise O, e.g. ahydroxyl group. An illustrative example is lactic acid.

Said at least one salt of a carboxylic acid is typically initiallypresent in said composition at a level of between 0.1M and 14M relativeto the initial solvent composition, most typically in the range 4M to10M. Where the at least one salt is a mixture of two or more salts ofcarboxylic acids, the concentration indicated is the sum of theconcentrations of the two or more salts.

It may be that the at least one salt of a carboxylic acid (e.g. a singlesalt of a carboxylic acid) is present at a concentration of greater than4M. It may be that the at least one salt of a carboxylic acid (e.g. asingle salt of a carboxylic acid) is present at a concentration ofgreater than 5M. It may be that the at least one salt of a carboxylicacid (e.g. a single salt of a carboxylic acid) is present at aconcentration of greater than 6M. In these embodiments, it may be thatthe salt of a carboxylic acid is a salt of a C₁₋₄ aliphatic carboxylicacids, that may be straight chained or branched. It may be that the saltof a carboxylic acid is a potassium salt of a C₁₋₄ aliphatic carboxylicacids, that may be straight chained or branched.

It may be that the capture composition further comprises a baseadditive. The base additive is a chemical species that is capable ofdeprotonating the carboxylic acid from which the salt is derived. Thus,the base additive is a species having a conjugate acid that has a higherpKa than the carboxylic acid from which the salt is derived. The baseadditive may have a pKa of between 5 and 14 as measured in a diluteaqueous solution. In these embodiments, it may be that the salt of acarboxylic acid is a salt of a C₅₋₈ aliphatic carboxylic acid, that maybe straight chained or branched. It may be that the salt of a carboxylicacid is a potassium salt of a C₅₋₈ aliphatic carboxylic acid that may bestraight chained or branched.

It may be that the base additive is present in the composition at alevel of ≤70% w/w relative to the total composition.

In instances where said at least one salt of a carboxylic acid is usedin combination with a base additive, said at least one salt of acarboxylic acid is typically present in the formulation at a level ofbetween 0.1M and 5M relative to the initial solvent composition, mosttypically in the range 0.1M and 3M.

The base additive may be a salt. It may be therefore, that the baseadditive is not a nitrogen base (e.g. does not comprise an amine).

The base additive may be a salt of a phenol said phenol being optionallysubstituted with from 1 to 5 groups selected from halo, C₁-C₄-alkyl,nitro, cyano, C₁-C₄-haloalkyl, O—C₁-C₄-haloalkyl, or additional aromaticrings to make a polycyclic phenol. The salt of a phenol may be a salt ofan unsubstituted phenol.

The base additive may be a carbonate. The carbonate may be a potassium,sodium, lithium, magnesium or calcium carbonate, or a combinationthereof.

Where the base additive is a salt (e.g. a carbonate or a salt of aphenol), the salt may comprise the same cationic counterion as the saltof the carboxylic acid (or the first carboxylic acid previouslydescribed) or may comprise an alternative cationic counterion. Thus, thebase additive may be a potassium salt.

The base additive may be an amine base. Suitable base additives may, forexample, be selected from aliphatic or alicyclic amino derivatives,including primary, secondary or tertiary amines. Suitable aliphatic oralicyclic amino compounds may, for example, be selected from aliphaticamines such as tetramethylethylenediamine (TMEDA),tetraethylmethanediamine (TEMDA), tetramethylmethanediamine (TMMDA),tetramethyl-1,3-diaminopropane (TMPDA) or triethylamine (TEA),hydroxyl-substituted aliphatic or alicyclic amino compounds, such asmonoethanolamine (MEA), diethanolamine (DEA), N-methyldiethanolamine(MDEA), 2-diethylaminoethanol (DEAE), 3-(diethylamino)-1,2-propanediol(DAPD) or 2-amino-2-methyl propan-1-ol (AMP), alicyclic amines such aspiperazine (PZ), morpholine, piperidine, pyrrolidine or1,8-diazabicyclo[5.4.0]undec-7-ene (DBU), or amino acids, for exampleglycine or related compounds.

The base additive may be present in the composition at a concentrationof <15 M. The base additive may be present at a concentration of <10 M.The base additive may be present at a concentration of <5 M. The baseadditive may be present at a concentration of >0.1 M or of >1 M. Forexample, the base additive may be present in the composition at aconcentration of between 0.1M and 15M relative to the initial solventcomposition, most typically in the range 4M to 10M

The inventors have found that salts of shorter chain carboxylic acids(e.g. C₁-C₄-aliphatic carboxylic acids) are capable of holding a largeramount of CO₂ in solution. This is due to the higher solubility of thesalts of shorter chain carboxylic acids in water. Salts of higher chaincarboxylic acids (e.g. C₅-C₈-aliphatic carboxylic acids) have a lowersolubility and therefore provide a lower capacity for holding CO₂ insolution. However, salts of higher chain carboxylic acids can provide afaster rate of CO₂ absorption than lower chain carboxylic acids. Theinventors have found that the capacity of solutions of salts of higherchain carboxylic acids can be increased without unduly reducing the rateof uptake and that this can be achieved either by incorporating salts ofsmaller chain carboxylic acids or by incorporating a base additive.

The solvent system may be water.

Said salt of a carboxylic acid may be supported by, or immobilised onto,a solid substrate.

Water is essential to the mechanism of acid gas capture both as a mediumto bring the reactants together and, in the case of carbon dioxidecapture, as a reagent. However excessive water levels can reduce captureperformance and efficiency. Typically, water is present in the capturecomposition at a level ≤70% w/w and most typically 50% w/w.

Typically, said acid gas is carbon dioxide. Carbon dioxide may becomprised in a carbon dioxide containing waste gas stream, for examplethe flue gases from a combustion process, or may be comprised in acarbon dioxide containing raw material gas stream, for example theproduct gas from an anaerobic digestion process. Other acid gases mayinclude sulfurous gases, such as hydrogen sulfide or sulfur dioxide, oroxidised nitrogenous gases commonly referred to as NO_(x).

Said capture composition is typically in a liquid form and mayoptionally comprise, for example, a slurry, a dispersion or asuspension. The viscosity of the capture composition is typically below50 cP. Typically, however, said composition comprises a solution whichgenerally has a total concentration of at least 1M. Typically,contacting the at least one acid gas with said composition mayconveniently be achieved by passing an acid gas containing gas streamthrough said composition, for example with a bubble tray column,alongside said composition, for example in a packed column, or using anyother process known to those skilled in the art.

The method of the invention is typically applied to the capture andrelease of carbon dioxide and is most conveniently carried out byinitially contacting the gas, typically carbon dioxide, with thecomposition in aqueous solution at typically −20° to 100° C., mosttypically 10° to 50° C. These are the initial temperatures of contactand the temperature may subsequently rise as a consequence of theexothermicity of the capture reaction.

Gas capture may be achieved at pressures in the range 0 to 300 Bara. Inspecific embodiments capture of gas may typically be achieved atpressures around 1 to 2 Bara; in alternative embodiments usingpressurised systems, capture of gas typically occurs at pressures in therange 0 to 30 Bara, most typically 20 Bara.

Release of the gas typically occurs at temperatures which are in therange of from 0° to 200° C., most typically in the range of 40° to 140°C., e.g. 60° to 100° C.

Release of the gas is conveniently achieved at pressures in the range offrom 0 to 150 Bara. In specific embodiments, release of gas maytypically be achieved at pressures of around 1 to 3.5 Bara; inalternative embodiments using pressurised systems, release of gastypically occurs at pressures in the range of from 1 to 30 Bara, mosttypically around 20 Bara. In some embodiments the release of the gasoccurs at a pressure in the range of 0 to 1 Bara.

A particularly useful aspect of the technology is that the releaseprocess will generate carbon dioxide or other released gases in enclosedand/or pressurised systems which will allow for increased pressure andthis should reduce further compression requirements for storageapplications, with important implications for reductions in overallenergy consumption for the complete capture and storage process.

Release of the captured gas from the composition via application of heatfacilitates regeneration of the composition, such that it may be usedfor further capture and release operations with further acid gases uponcooling.

Release of the captured gas from the composition via the application ofa stream of stripping gas, for example air, requires no furtherregeneration of the solvent before it can be recycled for furthercapture and release operations. Release of the captured gas via a streamof stripping gas may be further enhanced via the application of heat.

Release of the captured gas from the composition via the reduction ofpressure in the desorption apparatus requires no further regeneration ofthe solvent before it can be recycled except to return the solvent toabsorption pressure. Release of the captured gas via a reduction inpressure in the desorption apparatus may be further enhanced via theapplication of heat.

The methods of the invention utilise a composition that can be used as aphysical solvent as well as a chemical solvent. Embodiments thereforeuse two CO₂ absorption mechanisms, both a physical and a chemical one,working in tandem. This may reduce the overall energy intensity of theCO₂ separation process either by increasing the CO₂ capacity of thecomposition at a given absorption pressure, or by reducing the workingabsorption pressure necessary to achieve a given CO₂ capacity.

The methods of the invention are simple and economic to implement andprovides a major improvement over the methods of the prior art in termsof capture efficiency and overall process energy requirements whilstsignificantly improving on the environmental profile by using lesshazardous reagents under milder conditions thereby greatly reducingemissions and degradation issues.

The invention may also be as described in the following numberedparagraphs:

-   -   1. A method for the capture of at least one acid gas in a        composition, the release of said gas from said composition and        the subsequent regeneration of said composition for re-use, said        method comprising performing, in order, the steps of:        -   a. capturing an acid gas by contacting said acid gas with a            capture composition comprising at least one salt of a            carboxylic acid dissolved in a solvent system consisting            substantially of water;        -   b. releasing said at least one acid gas by heating the            loaded capture composition and/or by subjecting the loaded            capture composition to a stream of stripping gas and/or by            applying a lower pressure than employed for absorption;        -   c. regenerating the capture composition by cooling and/or            increasing the pressure.    -   2. A method as described in paragraph 1, wherein the at least        one salt of a carboxylic acid is a single salt of a carboxylic        acid.    -   3. A method as described in paragraph 2, wherein the single salt        is present at a concentration of greater than 4M, optionally 5M,        further optionally 6M.    -   4. A method as described in paragraph 3, wherein the single salt        is a salt of a C₁-C₄-aliphatic carboxylic acid, that may be        straight chained or branched.    -   5. A method as described in paragraph 1, wherein at least one        salt of a carboxylic acid is a mixture of two or more salts of        carboxylic acids.    -   6. A method as described in paragraph 5, wherein the mixture is        a mixture of at least one salt of a C₁-C₄ aliphatic carboxylic        acid, that may be straight chained or branched, and at least one        salt of a C₅-C₆ aliphatic carboxylic acids, that may be straight        chained or branched.    -   7. A method as described in paragraph 5 or paragraph 6, wherein        the mixture is a mixture of two or more salts derived from a        single carboxylic acid or a mixture of carboxylic acids with        differing cationic counterions.    -   8. A method as described in paragraph 1 or paragraph 2, wherein        the capture composition further comprises a base additive.    -   9. A method as described in paragraph 8, wherein the salt of a        carboxylic acid is a salt of a C₅₋₈ aliphatic carboxylic acids,        that may be straight chained or branched.    -   10. A method as described in paragraph 8 or paragraph 9, wherein        the base additive is a salt of a phenol said phenol being        optionally substituted with from 1 to 5 groups selected from        halo, C₁-C₄-alkyl, nitro, cyano, C₁-C₄-haloalkyl,        O—C₁-C₄-haloalkyl or additional aromatic rings to make a        polycyclic phenol.    -   11. A method as described in paragraph 8 or paragraph 9, wherein        the base additive is an amine base.    -   12. A method as described in paragraph 8 or paragraph 9, wherein        the base additive is a carbonate, optionally a lithium,        potassium, sodium, magnesium or calcium carbonate, or a        combination thereof.    -   13. A method as described in any preceding paragraph wherein        said salt of a carboxylic acid is a metal salt.    -   14. A method as described in paragraph 12 wherein said salt of a        carboxylic acid is a potassium salt.    -   15. A method as described in any preceding paragraph wherein        said at least one acid gas comprises carbon dioxide gas.    -   16. A method as described in any preceding paragraph wherein        said release of said at least one acid gas occurs at a        temperature in the range 60° to 100° C.    -   17. A method as described in any preceding paragraph wherein        said release of said at least one acid gas occurs at a pressure        in the range 0 to 1 Bara.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention are further described hereinafter withreference to the accompanying drawings, in which:

FIG. 1 illustrates the relationship between the maximum capture capacityof concentrated carboxylic acid salt solutions under 1 Bar partialpressure of CO₂ and the water:salt molar ratios found in said solutions.

FIG. 2 shows the improvement in CO₂ capture rate and capacity garneredthrough blending potassium hexanoate and potassium acetate as comparedto each component alone.

FIG. 3 illustrates the relationship between the number of carbon atomsin various carboxylic acid salts and the maximum observed rate of carbondioxide absorption under 1 bar partial pressure of CO₂.

FIG. 4 shows the improvement in rate of carbon dioxide absorption into a2M solution of potassium carbonate via the addition of 2M potassiumhexanoate.

DESCRIPTION OF THE INVENTION

The term ‘consisting substantially of water’ means that the solventsystem in which the carboxylic acid salt and any additional componentsare dissolved is substantially free of organic solvents. Thus, it may bethat the solvent system in which the carboxylic acid salt and anyadditional components are dissolved comprises at least 50% water, e.g.at least 75% water, 90% water, 95% water or 98% water, such as at least99% water. Thus, where the capture composition comprises only acarboxylic acid salt, the carboxylic acid salt is dissolved in a solventsystem that comprises at least 50% water, e.g. at least 75%, at least90% water, at least 95% water, 98% water, or at least 99% water. Wherethe capture composition comprises a base additive, the carboxylic acidsalt and the base additive are dissolved in a solvent system thatcomprises at least 50% water, e.g. at least 75% water, at least 90%water, at least 95% water, at least 98% water, or at least 99% water.

The term ‘solvent system’ refers to the mixture of liquids in which thecarboxylic acid salt and any base additive or other components aredissolved. The term ‘capture composition’ refers to the totalcomposition, i.e. both the solvent system and the carboxylic acid saltand any base additive or other components. Because the concentration ofthe carboxylic acid salt can be very high, the ‘capture composition’ maycontain only a small amount of the ‘solvent system’ and therefore only asmall amount of water. Nevertheless, the liquid in which the carboxylicacid is dissolved (i.e. the solvent system) will consist substantiallyof water.

The present inventors have provided a new system of acid gas capturecompositions that provides significant advantages over the methods ofthe prior art and finds potential applications in areas such as powerstations, cement manufacture, iron and steel manufacture, glass making,brewing, syngas processes, natural gas and biogas purification and otherchemical process such as ammonia production, hydrogen production,power-to-gas (e.g. Sabatier reaction) processes, as well as any otheracid gas producing industrial, commercial or domestic processes. In aparticular application, the defined method may be applied to the captureof carbon dioxide directly from the atmosphere.

As well as such applications, the compositions provided by the presentinvention are also appropriate for use in smaller scale specialistapplications such as, for example, in submarines, spacecraft and otherenclosed environments.

A particular embodiment of the invention envisages the application ofthe disclosed method to the capture and subsequent release of carbondioxide. The incorporation of carbon dioxide into a substrate is knownas carboxylation; the removal of carbon dioxide is known asdecarboxylation. This carboxylation/decarboxylation process is key toeffective carbon dioxide capture and capture formulation regeneration.

The present invention also envisages the use of the disclosedformulations in the capture and subsequent release of other acid gases,for example hydrogen sulfide or sulfur dioxide, in applications such asnatural gas sweetening, biogas upgrading and desulfurisation. Removal ofNO_(x) species from, for example, the waste gases from a combustionprocess may be an additional application.

In specific embodiments of the invention, systems are provided whereincarbon dioxide is captured by use of formulations which comprisepotassium acetate, potassium propionate, potassium iso-butyrate andpotassium hexanoate in water either solely or in combination with eachother or in combination with, for example, potassium carbonate orpotassium phenolate. The use of such systems facilitates the highlyefficient capture of the gas at ambient temperature and pressure.

Thus, the inventors have conducted a series of trials utilisingequipment adapted for the measurement of vapour-liquid equilibria (VLE).Specifically a system was provided which comprised a jacketed stainlesssteel vessel with the jacket connected to a temperature controlledcirculating bath and the vessel equipped with two temperature probes(one for monitoring vapour temperature, one for solvent temperature), a0-7 Bara pressure transducer, a safety release valve and emergencyrupture disc, a vacuum port for removing the internal atmosphere, acarbon dioxide inlet, an air inlet and gas entraining mechanicalstirring. Carbon dioxide is supplied from a temperature and pressuremonitored high pressure burette via a regulator to control the internalpressure of the VLE vessel and thus the partial pressure of carbondioxide.

TABLE 3 Maximum absorption rate and capacity for carboxylate saltsMaximum Absorption Maximum CO₂ Rate [Salt] Capacity (mol_(CO2)/ EntrySalt (mol/L) (mol/L) L/Bar/h) 1 Potassium acetate 8 1.24 2.5 2 Potassiumacetate 9 1.84 1.6 3 Potassium propionate 7 1.77 2.9 4 Potassiumiso-butyrate 7 1.97 2.5 5 Potassium valerate 4 0.37 6.2 6 Potassiumhexanoate 2.5 0.30 6.6 7 Potassium octanoate 2.4 0.45 3.5 150 mL ofaqueous solution in the VLE vessel under 1 Bar of CO₂ at initialtemperature of 20° C.

From the data collected (150 mL of aqueous solution in the VLE vesselunder 1 Bar of CO₂ at initial temperature of 20° C. Table 3), for agiven formulation, it is possible to determine the absolute rate ofcarbon dioxide absorption, the rate of carbon dioxide absorption as afunction of carbon dioxide partial pressure, the overall carbon dioxidecapacity as a function of temperature and/or carbon dioxide partialpressure. Combining data from a series of experiments for a particularformulation allows predictions to be made regarding the performance ofsaid formulation in a real process.

In order to demonstrate the highly effective nature of the systems,experimental campaigns were undertaken with a broad range of singlecarboxylate salts, mixed carboxylate salts and other capture agentsaccelerated by carboxylate salts all in aqueous solution.

From the data collected with the VLE, it is possible to draw a graph ofmaximum solution capacity vs. water:salt molar ratio (FIG. 1). As can beseen, there is a significant increase in the capacity of theseformulations once the water:salt molar ratio drops below approximately6. This supports the claim that the contribution of the salt to theoverall solvent character of the formulation is an important factor whendetermining the apparent pKa of carboxylate salts in such formulations.

The efficacy of stripping from these formulations is boosted by themechanism of acid gas absorption. Under these conditions, thethermodynamics of the reaction between hydrated carbon dioxide and thecarboxylates salts is more finely balanced than for more conventionalcapture agents, allowing regeneration to be less energy intensive.Experiments undertaken in the VLE with 7M potassium acetate in water ata variety of temperatures prove this to be the case. With a ΔT of just20° C. between the absorber and the desorber, in principle a cycliccarbon dioxide capacity of nearly 0.6 mol_(CO2)/L is possible (150 mL of7M KOAc solution in VLE under 1 Bar CO2 Table 4).

TABLE 4 Maximum CO₂ capacity of 7M KOAc solutions at differentabsorption temperatures Temperature Max Capacity Entry (° C.) (mol/L) 110 1.01 2 20 0.69 3 30 0.43 150 mL of 7M KOAc solution in VLE under 1Bar CO₂

Varying the pressure of carbon dioxide during absorption has aninfluence on both the absolute rate of carbon dioxide absorption as wellas the maximum capacity of the solution. To ensure that the hereindisclosed method is generally applicable, experiments have beenundertaken at partial pressures of carbon dioxide both above and below 1Bar (150 mL of solution in the VLE at 20° C. under pressure of CO2 Table5).

TABLE 5 Effect of CO₂ pressure on absolute absorption rate and capacityCO₂ Absolute Maximum Pressure Rate Capacity Entry Solution (Bar)(mM_(CO2)/s) (mol_(CO2)/L) 1 7M Potassium acetate 1 0.6 0.69 2 7MPotassium acetate 2 1.3 1.05 3 7M Potassium acetate 3 2.2 1.28 4 7MPotassium acetate 4 3.0 1.48 5 7M Potassium iso-butyrate 0.3 0.15 1.61 67M Potassium iso-butyrate 1.0 0.7 1.97 7 7M Potassium iso-butyrate 2.01.5 2.35 150 mL of solution in the VLE at 20° C. under pressure of CO₂

The variation in CO₂ capacity of these formulations as a function of CO₂partial pressure demonstrates that the formulations may be used asphysical solvents as well as chemical solvents. The advantage such asystem configuration would present would be to have two CO₂ absorptionmechanisms, both a physical and a chemical one, working in tandem. Thiswould reduce the overall energy intensity of the CO₂ separation processeither by increasing the CO₂ capacity of the working fluid at a givenabsorption pressure or by reducing the working absorption pressurenecessary to achieve a given CO₂ capacity.

Combining two or more salts in the same solutions can have unexpectedeffects. For example, a solution containing 1.5M potassium hexanoate and4M potassium iso-butyrate has a maximum capacity of 1.3 mol_(CO2)/L,significantly greater than either individual component or the sum oftheir individual capacities. Similarly, a blend of 2M potassiumhexanoate with 4M potassium acetate shows greater capacity than eithercomponent or the sum of its parts, at a greater absorption rate thanwould be expected from either component. This is demonstrated in FIG. 2.As can be extrapolated from FIG. 2, 2M potassium hexanoate would beexpected to have a capacity of about 0.2 to 0.25M CO₂ and 4M potassiumacetate a capacity of about 0.1 to 0.15M; while FIG. 2 indicates that ablend of 2M potassium hexanoate with 4M potassium acetate has a capacityof at least 1M CO₂.

There is a relationship between the total number of carbon atoms in thecarboxylate salt and the maximum observed rate. As can be seen, thispeaks at around five or six carbon atoms in the carboxylate salt (FIG.3).

This can be exploited by blending faster carboxylate salts with a secondcomponent that is basic enough to effect deprotonation of the carboxylicacid as it forms and holding the captured carbon dioxide as bicarbonatein solution. Specific examples include carbonate and phenolate salts of,for example, lithium, sodium, potassium, calcium or magnesium. Adding 2mol/L potassium hexanoate to a 2M solution of potassium carbonate haslittle impact on the overall capacity of the solution but increases therate of carbon dioxide absorption by more than a factor of two (FIG. 4).

Similarly, a solution consisting of 2.5M potassium hexanoate and 1Mpotassium phenolate exhibits a faster rate of absorption (+25%) andgreater capacity (+100%) as compared to a 2.5M solution of potassiumhexanoate alone.

This accelerating effect extends to other capture agents that mightotherwise be considered “slow”. Adding 1M potassium hexanoate to a 5Msolution of 2-amino-2-methyl-1-propanol (AMP) increases the rate ofcarbon dioxide absorption by approximately 30%. This effect remainsafter a full absorption and release cycle for a second absorption.

Carboxylate solutions themselves can have their rate of carbon dioxideabsorption accelerated by the usual accelerants known to those skilledin the art. For example, including 250 mg/L of carbonic anhydrase in a7M solution of potassium acetate increases the rate of carbon dioxideabsorption by approximately 15%.

The invention claimed is:
 1. A method for the capture of at least oneacid gas in a composition, said method comprising: a. capturing an acidgas by contacting said acid gas with a capture composition comprising ametal salt of a C₅-C₈-aliphatic carboxylic acid and an additivedissolved in a solvent system to form a loaded capture composition,wherein said solvent system comprises at least 95% water, wherein thesolvent system is a mixture of liquids in which the carboxylic acid saltis dissolved, wherein said capture composition comprises no more than 1%of amino acids or salts of amino acids, wherein said acid gas comprisescarbon dioxide; wherein the additive is selected from a carbonate saltand a metal salt of a C₁-C₄-carboxylic acid.
 2. A method of claim 1,further comprising performing, in order, the steps of: b. releasing saidat least one acid gas by heating the loaded capture composition and/orby subjecting the loaded capture composition to a stream of strippinggas and/or by applying a lower pressure than employed for absorption; c.regenerating the capture composition by cooling and/or increasing thepressure.
 3. A method as claimed in claim 2 wherein said release of saidat least one acid gas occurs at a temperature in the range 60° to 100°C.
 4. A method as claimed in claim 2 wherein said release of said atleast one acid gas occurs at a pressure of 1 Bara or less.
 5. A methodas claimed in claim 1, wherein the mixture capture composition is amixture of at least one metal salt of a C₁-C₄ aliphatic carboxylic acid,that may be straight chained or branched, and at least one metal salt ofa C₅-C₆ aliphatic carboxylic acid, that may be straight chained orbranched.
 6. A method as claimed in claim 1, wherein the base additiveis a carbonate.
 7. A method as claimed in claim 6, wherein the carbonateis lithium, potassium, sodium, magnesium or calcium carbonate, or acombination thereof.
 8. A method as claimed in claim 1 wherein saidmetal salt of a C₅-C₈ aliphatic carboxylic acid and/or C₁-C₄ carboxylicacid is a potassium salt.
 9. A method as claimed in claim 1, wherein theacid gas further comprises hydrogen sulphide or sulphur dioxide.
 10. Themethod of claim 1, wherein the single metal salt of a C₅-C₈-aliphaticcarboxylic acid is present at a concentration of greater than 5 M. 11.The method of claim 1, wherein the single metal salt of aC₅-C₈-aliphatic carboxylic acid is present at a concentration of greaterthan 6 M.